Carbon dioxide is an undesired impurity in many commercial gas applications owing to its ability to freeze and form hydrates with moisture at low temperatures. Formation of solids or solid particles makes gas processing, operation, transportation and storage quite difficult or even impossible. For example, cryogenic units for air separation to produce oxygen and nitrogen demand practically complete removal of carbon dioxide (1 ppm and less) and water vapor from air before separation. Refineries place similar requirements on the quantity of carbon dioxide and moisture present in hydrogen-contaminated gas flows. Similar requirements are placed on ammonia plants for nitrogen purity and gas processing plants on the carbon dioxide content and natural gas dew point prior to ethane and helium recovery and/or before natural gas liquefaction. Also, petrochemical plants have to eliminate carbon dioxide and water in monomers: ethylene, propylene, butadiene, etc. to prevent poisoning of the polymerization catalysts and deterioration of polymer properties.
Adsorption of carbon dioxide and water vapor is the most common method of removal of these compounds from gas streams due to high performance and relatively low capital and operational costs. Two adsorption techniques are commonly used in commercial gas manufacturing: temperature swing adsorption (TSA) and pressure swing adsorption (PSA). Efficiency of both adsorption processes is determined by the properties of the adsorbent. High adsorption of carbon dioxide is the most important property of the adsorbent, especially at very low partial pressures.
Several types of CO.sub.2 adsorbents have been created to meet industry needs. Because general duty adsorbents, such as alumina, silica gel and activated carbon, do not have a substantial adsorption capacity for carbon dioxide, more complex adsorbents have been prepared.
U.S. Pat. No. 3,865,924, Gidaspow, discloses a carbon dioxide adsorbent, which constitutes a mechanical mixture of activated alumina and alkali metal carbonates. U.S. Pat. No. 4,433,981, Slaugh, discloses an adsorbent prepared by impregnating alumina with an alkali or alkaline earth metal oxide or salt decomposable upon calcination. U.S. Pat. No. 4,493,715, Hogan, discloses an adsorbent for carbon dioxide removal from olefins which comprises alkali metal oxides, hydroxides, nitrates, acetates, etc. placed on an activated alumina.
All such adsorbents employ chemisorption or reversible chemical reactions to bind carbon dioxide to the metal carbonates or bicarbonates. The main disadvantage of these adsorbents is low operational reliability and short life due to the tendency of active components to sinter. Secondly, the time before water breakthrough on the majority of adsorbents is shorter than the time before the carbon dioxide breakthrough. This results in the need to employ supplemental desiccant beds.
It is also impossible to use base-containing adsorbents in the PSA-type units because they form compounds with CO.sub.2 that do not regenerate under reduced pressure.
A newer process for gas dehydration and carbon dioxide recovery technology uses molecular sieves, natural and synthetic zeolites. It is known that synthetic zeolite A and X types are effective adsorbents of CO.sub.2 and water. For instance, U.S. Pat. No. 3,981,698, Leppard, U.S. Pat. No. 4,039,620, Netteland, U.S. Pat. No. 4,711,645, Kumar, U.S. Pat. No. 4,986,835, Uno, and U.S. Pat. No. 5,156,657, Ravi, suggest the use of standard molecular sieves 5A, 10A and 13X as carbon dioxide adsorbents. These molecular sieves adsorb CO.sub.2 by physical adsorption and are regenerable at ambient temperatures. However, they do not possess sufficient adsorption capacity for carbon dioxide. Thus, such adsorbents cannot provide extensive gas purification, demand an increased loading volume and often require the use of supplemental adsorbent beds to decrease the water and carbon dioxide concentration prior to introduction into the zeolite bed.
To increase carbon dioxide adsorption capacity, several adsorbents have been proposed based on various cation exchanged forms of molecular sieve X and other crystalline structures. Thus, U.S. Pat. No. 3,885,927, Sherman, discloses a barium cation form of zeolite X in which 90-95% of the Na.sup.+ ions are replaced by Ba.sup.2+ ions. U.S. Pat. No. 4,477,267, Reiss, utilizes an adsorbent for hydrogen purification containing CaX-zeolite. For carbon dioxide removal, U.S. Pat. No. 4,775,396, Rastelli, describes the use of zinc, rare earth metals, a proton and ammonium cation exchanged forms of synthetic faujasite having a silica: alumina ratio in broad range of 2-100. U.S. Pat. No. 5,587,003, Bulow, discloses the use of a natural or synthetic clinoptilolite, which contains as exchangeable cation the ions of metals of Groups 1A, 2A, 3A, 3B, the lanthanide group and mixtures of these.
All of these molecular sieve adsorbents are characterized by carbon dioxide adsorption capacity extended at moderate and high partial pressures of the admixture to be adsorbed. However, their capacity to adsorb at low partial pressure of CO.sub.2 (&lt;5 torr) and at ambient temperatures is not sufficient to provide the purity of the gas required. In addition, due to the relatively short time before CO.sub.2 breakthrough, the water capacity of these adsorbents appears to be only 10-15 percent of potential. This decreases adsorbent performance in such applications as TSA and PSA air pre-purification units where carbon dioxide inlet adsorption is very low. Employing the above mentioned adsorbents in such applications demands gas chilling to a temperature below about 5.degree. C. In turn, this results in a substantial increase in operation and capital costs.
U.S. Pat. No. 5,531,808, Ojo, discloses an adsorbent for carbon dioxide adsorption comprising a type X zeolite having a silicon to aluminum ratio in the range of 1.0-1.15. The type X zeolite adsorbent contains ions of Group 1A, Group 2A, Group 3A, Group 3B, the lanthanide series and mixtures thereof. It fails to teach any critical quantitative relationship among various cations in the type X-zeolite crystalline structure that is necessary to provide high levels of adsorption capacity of carbon dioxide at low partial pressures and at ambient temperatures. It also fails to disclose preferable limits for crystalline purity and crystal sizes. It also does not disclose the adsorbent macroporosity limits necessary to provide the appropriate kinetics and dynamics of carbon dioxide adsorption.
A process for preparing low-silica faujasite (LSF) with a silica/alumina ratio .about.2.0 is disclosed in Kuhl "Crystallization of Low-Silica Faujasite" Zeolites, vol. 7 p.451 (1987). Kuhl discloses that both sodium and potassium cations should be present to obtain faujasite crystals with relatively low silica content. The crystallization process disclosed comprises preparing a sodium aluminate water solution with addition of sodium and potassium hydroxides, mixing the solution with sodium silicate, aging the gelled mixture, and filtering and washing of the crystallization product. Kuhl also describes specific reagent ratios, temperatures and retention times, which are required for crystallization of the product. However, it does not specify the range of crystallization parameters that provide definite size of the faujasite crystals and a final product with a low content of admixture crystals of different types. Kuhl also does not disclose sodium-potassium ion exchange procedures for obtaining LSF with low residual potassium ion content.
A number of other patents disclose molecular sieve adsorbents having improved adsorption capacities, especially for the removal of carbon dioxide from gas mixtures. For example, U.S. Pat. No. 2,882,244, Milton, discloses a variety of crystalline aluminosilicates useful for CO.sub.2 adsorption. U.S. Pat. No. 3,078,639, Milton, discloses a zeolite X useful for the adsorption of carbon dioxide from a gas stream. British Patent Nos. 1,508,928, Mobil Oil, and 1,051,621, Furtig et al., disclose faujasite-type zeolites having a silica to alumina ratio from 1.8 to 2.2.
While these products have been useful in the adsorption of carbon dioxide and water from gas streams, it is important to provide improved adsorbents. Further, while it has been discovered that low silica faujasites are useful in the adsorption of carbon dioxide and water from gas streams, newer low silica faujasites with improved adsorption capabilities which do not exhibit the limitations of the earlier products would be helpful.
Accordingly, it is an aspect of the invention to provide an adsorbent for carbon dioxide and water vapor with enhanced adsorption capacity.
It is a further aspect of the invention to provide an adsorbent for carbon dioxide useful for adsorption at ambient temperatures and low partial pressures.
It is a still further aspect of the invention to provide an adsorbent for carbon dioxide in a gas stream which reduces operation and capital expenses when used.
It is a still further aspect of the invention to provide an adsorbent for carbon dioxide and water vapor with improved kinetics and dynamics of adsorption for both Temperature Swing Adsorption processes and Pressure Swing Adsorption processes and an aggregate of the two processes.
It is a still further aspect of the invention to disclose an adsorbent for carbon dioxide which produces a gas stream containing less than one part per million of carbon dioxide.
It is a still further aspect of the invention to provide a process for the production of a low silica faujasite adsorbent for carbon dioxide.
These and further aspects of the invention will be apparent from the foregoing description of a preferred embodiment of the invention.